sd_journal_next() advances the read
pointer into the journal by one entry. The only argument taken is
a journal context object as allocated via
sd_journal_open(3).
After successful invocation the entry may be read with functions
such as
sd_journal_get_data(3).

Similarly, sd_journal_previous() sets
the read pointer back one entry.

sd_journal_next_skip() and
sd_journal_previous_skip() advance/set back
the read pointer by multiple entries at once, as specified in the
skip parameter.

The journal is strictly ordered by reception time, and hence
advancing to the next entry guarantees that the entry then
pointing to is later in time than then previous one, or has the
same timestamp.

Note that
sd_journal_get_data(3)
and related calls will fail unless
sd_journal_next() has been invoked at least
once in order to position the read pointer on a journal
entry.

Note that the SD_JOURNAL_FOREACH()
macro may be used as a wrapper around
sd_journal_seek_head(3)
and sd_journal_next() in order to make
iterating through the journal easier. See below for an example.
Similarly, SD_JOURNAL_FOREACH_BACKWARDS() may
be used for iterating the journal in reverse order.

The four calls return the number of entries advanced/set
back on success or a negative errno-style error code. When the end
or beginning of the journal is reached, a number smaller than
requested is returned. More specifically, if
sd_journal_next() or
sd_journal_previous() reach the end/beginning
of the journal they will return 0, instead of 1 when they are
successful. This should be considered an EOF marker.

All functions listed here are thread-agnostic and only a single thread may operate
on a given sd_journal object.

The sd_journal_next(),
sd_journal_previous(),
sd_journal_next_skip() and
sd_journal_previous_skip() interfaces are
available as a shared library, which can be compiled and linked to
with the
libsystemdpkg-config(1)
file.